It is known, at least to reduce the nitrogen oxides contained in exhaust gases of internal combustion engine vehicles by means of a selective catalytic reduction system (SCR procedure). For this purpose an aqueous urea solution will be injected into the exhaust stream in fine dosing steps to reduce the nitrogen oxides. Due to a chemical reaction with the urea solution, the nitrogen oxides will be transformed into nitrogen and water. Pumps are used for pumping the urea solution, like piston pumps, rubber diaphragm pumps, gear pumps, orbital and hose pumps. They are however not robust, so they do not have a long life when used in SCR systems in vehicles. Also, precise dosage of the urea solution is likely very difficult to inject into the exhaust gas stream and hardly possible with them. But, precise dosage is required for a reliable reduction of nitrogen oxides in the exhaust stream.